Spinning rear tires

[xr4x4ti]

Hey SyTy world.

I am the guy using a SyTy transfer case and front diff in a Turbo V8 AWD Merkur XR4Ti. For the last few years I have been turning the car into more of a race car. Or, at least making changes to make it better at the drag strip. I have had the car on the rode with the V8 AWD setup for over 10 years now and as I am sure most SyTy owners will understand, the thing is grip machine on the street. I know I am preaching to the choir here, but every time I give a AWD denier a ride they soon become a believer

But, I find that when I really push it at the track with aggressive but not crazy launches that I an get the rear tires to spin for a bit. I recently made some changes I thought would solve this problem and I went to the track yesterday to try them out. I put on some really sticky tires, Bridgestone RE-71R's and put in a fresh Viscous Coupling. I also put the limited slip in the front that was recently made available since I also occasionally spin 3 wheels. But yesterday I still manged to spin the rear tires.

Have others experienced this?

After this happened last fall I did a bit of research on viscous coupling and I realized they don't work quite the way I always thought they did. I believe the kind of VC used in our TC never truly locks up and can only transfer about 100 ft-lbs or less across the center diff. This is completely adequate for a stock AstroVan with 200hp, but I am pushing 600 to the wheels and now I just don't think it can keep up.

Thoughts? Has anybody ever tested a VC using a torque wrench in a lathe to see with the torque VS RPM curve looks like?

I have spoke with STE http://stecouplers.com/
.
They can make a custom coupling that that is stiffer, but I need to do some baseline testing first.

My other thought is to go with a Trailblazer SS NV120 TC. This is a Torsen style and wouldn't have any lag, but has other issues I can get into details about later.

Thanks in advance,
Tim
XR4X4Ti

 

[dgoodhue]

Re: Spinning rear tires

I regards to wheel spinning, are you talking about braking torquing while the truck is stopped? Or off the launch?

 

[xr4x4ti]

Re: Spinning rear tires

I regards to wheel spinning, are you talking about braking torquing while the truck is stopped? Or off the launch?

No, the car is a stick, so I don't "brake torque" it. I am talking after the light turns green punching it and I can spin the rear tires. Of course my rear suspension is different from the SyTy's and probably doesn't hook as well, but I would hope that the TC would try to prevent the wheel spin my transferring more of the torque to the front wheels. I guess in an ideal world I would think that all four wheels would brake loose at the some time.

 

[dgoodhue]

Re: Spinning rear tires

I don't know what you have for a rear end, but the factory SyTy uses a gov lock rear end that unlocks supposedly around 35mph.

Another potential is perhaps the viscous coupler in your truck failing open. (It's seems as most failed locked though). If It is slipping a viscous coupler fluid should gell and provide a nearly 50:50 split. You can test a the viscous coupler by locking the rear wheels (park, ebrake), removing the front prop shaft and putting a torque wrench on the nut propelling the front wheels.

 

[dgoodhue]

Re: Spinning rear tires

Btw I don't think 600 to wheels is a problem for the SyTy transfer case. 8 and 9 sec truck with much more than 600 whp are using it.